These fine gents beat me to it, but as I took the pics and put in the effort I'll still go into detail lol.
As mentioned above all models have a 360 degree arc of sight and can turn at any time for free
So there's no difference between this:
and this
Spears as stand alone units do not gain any advantage, but they may contribute an attack to a combat by being in base contact with a friendly model who is already in combat. Like this:
In this way spears can really help against cavalry as they can add an additional attack against the charging cavalry (who also have gained an extra attack as they do this when charging).
However your original question asks about facing the wrong way, and that is possible, in the following picture the spear armed model is the only one who fights as the sword model can only fight if directly in contact with the enemy. So you are losing the advantage of a spear. And in effect your formation is facing the wrong way.
Hope that helps
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